# jay_diff — JSON diff / patch library `utils/jay_diff.py` is a lightweight, pure-Python library for computing and applying structural diffs between two JSON-compatible dicts. It is used by `collect.py` and `gui_client.py` to compress the push-server stream: instead of retransmitting full snapshots on every poll, the server sends only the fields that changed since the previous snapshot. --- ## Concepts A **diff** is itself a plain dict. It describes what changed from an **old** state to a **new** state using one or more of these keys: | Key | Meaning | |-----|---------| | `update_add` | Fields whose value changed *or* that are new in `new` | | `update` | Fields whose value changed (must already exist in `old`) | | `add` | Fields that are new in `new` (did not exist in `old`) | | `delete` | Fields that were removed or whose sub-keys were pruned | `update_add` combines `update` and `add` into a single key (the mode used everywhere in this project). The separate `update` / `add` form is available via `combine_upd_add=False` for completeness. An **empty dict `{}`** returned by `jay_diff_full` means the two states are identical. ### Delete-delta encoding Values inside the `delete` sub-dict carry a specific meaning: | Value in delete delta | Effect when merged | |-----------------------|--------------------| | `None` | The **entire key** is deleted from `old` | | A **non-empty dict** | Recurse into the sub-dict and delete those specific sub-keys; the parent key is kept (possibly as `{}`) | | A **scalar** (string, int, …) | The key is deleted from `old` | | `{}` (empty dict) | The key is deleted from `old` | This distinction matters when the new state has an empty dict `{}` at a location where the old state had a populated dict. In that case the delete delta carries the populated sub-keys as a non-empty dict — after merging, the parent key is preserved as `{}`. If the key is entirely absent from the new state, `None` is stored in the delta, and the merge removes the key completely. --- ## API ### Diff functions ```python jay_diff_full(old, new, combine_upd_add=True) -> dict ``` The main entry point. Computes a complete diff from `old` to `new`. - `combine_upd_add=True` (default) — produces `update_add` + `delete` - `combine_upd_add=False` — produces `update` + `add` + `delete` Returns `{}` when `old == new`. ```python jay_diff_add(old, new) -> dict ``` Returns keys present in `new` but absent from `old` (additions only). ```python jay_diff_del(old, new) -> dict ``` Returns keys present in `old` but absent from `new`, or sub-keys that were removed from a shared dict. Uses `None` as sentinel for entirely-absent dict keys. ```python jay_diff_upd(old, new) -> dict ``` Returns keys present in both `old` and `new` whose values differ (updates only; new keys are ignored). ```python jay_diff_upd_add(old, new) -> dict ``` Like `jay_diff_upd` but also includes keys new in `new` — equivalent to `jay_diff_full(..., combine_upd_add=True)` minus the delete section. --- ### Merge functions ```python jay_merge_full(old, diff) -> dict ``` The main entry point. Applies a diff produced by `jay_diff_full` to `old` and returns the reconstructed new state. **Bootstrap passthrough:** if `old` is `{}` and `diff` contains none of the recognised diff keys, `diff` is returned as-is. This is how the first message from the push server is handled: the server sends `jay_diff_full({}, record[0])`, which looks like a plain snapshot, and the client stores it directly. Applies operations in order: `update` → `update_add` → `add` → `delete`. ```python jay_merge_update(old, new) -> dict # apply an update/update_add delta jay_merge_add(old, new) -> dict # apply an add delta jay_merge_delete(old, new) -> dict # apply a delete delta ``` Low-level helpers; prefer `jay_merge_full` unless you need fine-grained control. --- ## Round-trip guarantee ``` jay_merge_full(old, jay_diff_full(old, new)) == new ``` This holds for all inputs where values are JSON-compatible scalars (strings, numbers, booleans, `None`, lists) or nested dicts thereof. It is verified by `tests/test_jay_diff_real_data.py` against 674 consecutive pairs of real vehicle snapshots. --- ## Wire format `jay_diff_full` / `jay_merge_full` are used by the push server when `--diff` is enabled. The diff dict is serialised as a newline-delimited JSON line. - The **first** message to a new client is `jay_diff_full({}, record[0])`, which always contains `update_add` with every field of the first record. - Subsequent messages contain only the changed fields. - The format is self-describing: a message with none of the diff keys is treated as a full snapshot by `jay_merge_full` (bootstrap passthrough). `null` in JSON round-trips to Python `None`, so the delete sentinel survives serialisation transparently. --- ## Tests ``` tests/test_jay_diff.py — 104 unit tests (all diff/merge functions, corner cases: type changes, empty dicts, deep nesting, round-trips) tests/test_jay_diff_real_data.py — parametrised over 675 real VW snapshots in tests/data/ (674 consecutive pairs): round-trip, identical diff = {}, and empty-diff-is-noop checks ``` Run with: ```bash pytest tests/ ``` --- ## Bug history ### `jay_merge_delete`: crash on aliased dicts (fixed) `jay_diff_rev` (used internally by the old `jay_diff_del`) stored references to sub-dicts of `_old` directly in the delete delta without copying them. When `jay_merge_full(old, jay_diff_full(old, new))` was called, some sub-dicts in the delete delta were the same Python objects as sub-dicts in `old`. Iterating `_new.items()` while deleting from `_old` (the same object) raised `RuntimeError: dictionary changed size during iteration`. **Fix:** `list(_new.items())` snapshots the items before the loop. Also, `jay_diff_del` was rewritten as a standalone recursive function that never stores aliases to its inputs. ### `jay_merge_delete`: empty dict wrongly pruned (fixed) When the new state had an empty dict `{}` at a location where the old state had a non-empty dict, `jay_merge_delete` would delete all sub-keys and then prune the now-empty parent key entirely. The result differed from `new`, which retained the parent key as `{}`. Root cause: the old delta format stored the full old sub-dict for both "key absent from new" and "key present but empty in new" — the two cases were indistinguishable at merge time. **Fix:** `jay_diff_del` now stores `None` for dict keys entirely absent from `new`, and a non-empty sub-dict for partial deletions. `jay_merge_delete` no longer prunes: a `None` (or scalar) value in the delta is the explicit signal to remove the parent key.